The present invention relates to an image-display device having a faceplate formed with a phosphor layer and means irradiating an electron beam to the phosphor layer. More specifically, the present invention relates to an image-display device using, as a phosphor forming a phosphor layer, a ZnS phosphor having an improved luminescence efficiency.
At present, high resolution and large-screen display devices are being studied and developed increasingly in image information systems. Displaying a sharp image on a large screen is desired strongly to color display devices. The luminescence efficiency and color coordinates of display devices must be improved.
ZnS phosphors are used as a green luminescence phosphor and a blue luminescence phosphor for a cathode-ray tube such as a color picture tube or a color display tube and are a typical phosphor material used as a blue luminescence phosphor in a projection Braun tube. From the request to the display devices, the performance of the ZnS phosphors is required to be further improved.
To improve the luminescence efficiency and color coordinates, improvement in phosphor material composition and development of phosphor particle surface treatment methods have been performed. As a method for using a ZnMgS phosphor which is made into a compound crystal by adding Mg as a IIA element to ZnS, Japanese Published Unexamined Patent Application No. Hei 3-207786 reports ZnMgS:Pr3+ for an EL luminescence element. MgS is solid solved into ZnS using a high purity Mg metal, thereby facilitating incorporation of Pr3+ into a crystal.
Japanese Published Unexamined Patent Application No. Hei 6-299149 reports a ZnS:Ag phosphor provided on its surface with a barrier layer ZnMgS so as not to move carriers onto a surface layer having a large proportion of non-radiative recombination center.
J. Electrochem. Soc. 99 (1952) 155 reports a shift in short wave length of a luminescence spectrum by Mg under electron beam excitation in the range of Mg=5-30 mol % for ZnMgS:Cu,Cl and ZnMgS:Ag,Cl phosphors.
A Cl compound is used as a flux at phosphor synthesis. Green luminescence and SA luminescence (self-activate luminescence) as the cause of cross contamination are observed in the ZnMgS:Cu,Cl phosphor. The luminescence efficiency of the Cu and Ag activate phosphors has not been improved.
In a technical field entirely different from the phosphor, as a studied result of a crystal structure, J. Materials. Science 3 (1984) 951 reports lattice expansion in a MgSxe2x88x92ZnS compound crystal form due to increased Mg, change from a cubic form to a hexagonal form of a crystal structure at below 1020xc2x0 C., and limit of solid solution of MgS to ZnS.
As a novel phosphor manufacturing method, for the ZnS:Cu,Al phosphor, as in Japanese Published Unexamined Patent Application No. Hei 4-11687, there is a method for obtaining a phosphor of resistance to high electron beam excitation and high luminescence by specifying the mol ratio of Cu and Al as an activator.
As a method for using a hexagonal form ZnS phosphor, for the ZnS:Ag,Al phosphor, as in Japanese Published Unexamined Patent Application No. Hei 6-322364, there is a method for improving the luminescence tone and linearity by mixing a cubic form and a hexagonal form.
There is a method for obtaining a blue luminescence phosphor whose luminescence and electric current characteristics are improved to some extent using a hexagonal form of ZnS:Ag,M,Al phosphor (M is Cu or Au), as in Japanese Published Unexamined Patent Application No. Hei 11-349937.
To improve the luminescence efficiency of the ZnS phosphor, various methods have been studied. These prior art methods, however, cannot solve all the problems. In particular, in the green luminescence ZnS:Cu,Al and the blue luminescence ZnS:Ag,Al phosphors, Cu and Ag of a IB element added as radiative recombination center enter into an interstitial site without Zn site substitution to cause high energy luminescence such as Blue-Cu. It is one factor color un-coordinating green luminescence and inhibiting improvement in luminescence efficiency.
An object of the present invention is to provide an image-display device having an excellent luminescence characteristic by improving a luminescence energy efficiency and color coordinates by cathode-ray tube excitation which is the problem of the above prior art ZnS phosphor.
The above object is achieved by an image-display device having a faceplate formed with a phosphor layer, and means irradiating an electron beam to the phosphor layer, including: the phosphor layer consisting of a ZnS phosphor which is expressed by a general formula: Zn(1xe2x88x92x)MIIAxS:MIB,MIII where MIIA is at least one IIA element selected from the group of Be, Mg, Ca, Sr and Ba; MIB is at least one IB element selected from the group of Cu, Ag and Au; MIII is a III element including at least one of Al and Ga; and a solubility x is 0 less than x less than 0.25.
As one aspect of the ZnS phosphor used in the image-display device of the present invention, the ZnS host crystal lattice is expanded so that at least one IB element selected from the group of Cu, Ag and Au as the radiative recombination center (activator) facilitates Zn site substitution.
The Cu covalent radii r=0.135 nm and the Ag covalent radii r=0.152 nm are both larger than the Zn covalent radii r=0.131 nm. It is desirable to expand the host crystal lattice of the ZnS phosphor in order to improve the luminescence efficiency by increasing an amount of the IB element added to enhance the radiative recombination center density.
In the present invention, to expand the host crystal lattice, at least one IIA element selected from the group of Be, Mg, Ca, Sr and Ba is added in a suitable amount to ZnS.
In the case of Mg, the ion bonding properties are strong and the ionic radii are r=0.071 nm to be smaller than the Zn ionic radii r=0.074 nm. The host crystal lattice of ZnMgS as a compound crystal of ZnS and MgS is expanded as compared with ZnS. For example, when Mg is 30 mol %, it is expanded by about 0.005 nm in the a axis direction and by about 0.003 nm in the c axis direction. For the ionic radii of other IIA elements, Be: 0.034 nm; Ca: 0.106 nm; Sr: 0.127 nm; and Ba: 0.143 nm. Ca and Sr have the effect for singly expanding the ZnS lattice as in Mg.
When the different ITA elements are combined to be used, the lattice can also be expanded. A ZnMgCaS host crystal lattice having a combination of Mg and Ca and a ZnBeSrS host crystal lattice having a combination of Be and Sr are taken as an example.
The ionic radii of Be are much smaller than the Zn ionic radii and the ionic radii of Ba are too large. When incorporating the elements, they are desirably combined with at least one element of Mg, Ca and Sr for co-existence.
The solubility x of Zn and at least one IIA element selected from the group of Be, Mg, Ca, Sr and Ba is 0 less than x less than 0.25. Even when the lower limit value is small, it is found to improve the luminescence efficiency. It is preferably x=0.0001 or above.
The upper limit is x less than 0.25. The upper limit of the solubility x is different depending on the type of at least one IB element selected from the group of Cu, Ag and Au. With the large ionic radii one like Au, the upper limit is increased. It is also different depending on the calcination temperature of the phosphor. When the calcination temperature is increased, the allowable upper limit of the x value tends to be large.
MIII used as a co-activator is a III element including at least one of Al and Ga. Addition of In, Sc and Y as other III elements is allowable.
The Zn(1xe2x88x92x)MgxS phosphor obtained by adding Mg, one of IIA elements, to the host crystal can easily synthesize the phosphor of a crystal structure expressed by a composition formula xcex1axcex2(1xe2x88x92a) as compared with the prior art. xcex1 expresses a hexagonal form, xcex2 expresses a cubic form, and a expresses a solubility.
In the prior art, such compound crystal form is subjected to only temperature control. ZnS is transited in the narrow temperature range around about 1020xc2x0 C. In this case, since the solubility is changed by a slight temperature difference, it is difficult to synthesize a phosphor having a fixed solubility a. When using Zn(1xe2x88x92x)MgxS as a compound crystal with Mg, the compound crystal range of xcex1 and xcex2 is large. The Mg solubility x and calcination temperature T are determined to synthesize the phosphor having a fixed solubility a more easily than the prior art.
The bandgap of the hexagonal form (xcex1) is about 0.1 eV larger than the Cubic form (xcex2). The solubility a is changed in the range of 0 less than a  less than 1 to select the luminescence color coordinates of the phosphor.
The Zn(1xe2x88x92x)MgxS changes the solubility x to vary the luminescence color coordinates of the phosphor. Mg is added to the host crystal to increase the bandgap. The solubility x of the Zn(1xe2x88x92x)MgxS is determined to easily select the color coordinates.